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      <h1 class="post-title">JAVA生成安全随机数</h1>

      <div class="post-meta">
        <span class="post-time"> 2020-11-04 </span>
        <div class="post-category">
            <a href="/categories/java/"> java </a>
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    <nav id="TableOfContents">
  <ul>
    <li>
      <ul>
        <li><a href="#示例代码">示例代码</a></li>
        <li><a href="#问题">问题</a>
          <ul>
            <li><a href="#原因">原因</a></li>
            <li><a href="#解决方案">解决方案</a></li>
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    <div class="post-content">
      <h2 id="示例代码">示例代码</h2>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
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<td class="lntd">
<pre class="chroma"><code class="language-java" data-lang="java"><span class="kn">package</span> <span class="nn">cn.lglspace.stulab.util</span><span class="o">;</span>

<span class="kn">import</span> <span class="nn">java.security.NoSuchAlgorithmException</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">java.security.SecureRandom</span><span class="o">;</span>

<span class="kn">import</span> <span class="nn">org.apache.commons.lang3.RandomStringUtils</span><span class="o">;</span>

<span class="kd">public</span> <span class="kd">class</span> <span class="nc">RandomUtil</span><span class="o">{</span>
	
<span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">SecureRandom</span> <span class="n">SECURE_RANDOM</span><span class="o">;</span>
    
    <span class="kd">static</span> <span class="o">{</span>
        <span class="n">SECURE_RANDOM</span> <span class="o">=</span> <span class="n">getSecureRandom</span><span class="o">();</span>
    <span class="o">}</span>

    <span class="kd">private</span> <span class="nf">RandomUtil</span><span class="o">()</span> <span class="o">{</span>
    <span class="o">}</span>
    
    <span class="kd">private</span> <span class="kd">static</span> <span class="n">SecureRandom</span> <span class="nf">getSecureRandom</span><span class="o">()</span> <span class="o">{</span>
    	<span class="k">try</span> <span class="o">{</span>
    		<span class="c1">// 获取高强度安全随机数生成器
</span><span class="c1"></span>    		<span class="k">return</span> <span class="n">SecureRandom</span><span class="o">.</span><span class="na">getInstanceStrong</span><span class="o">();</span> 
        <span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="n">NoSuchAlgorithmException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
        	<span class="c1">// 获取普通的安全随机数生成器
</span><span class="c1"></span>            <span class="k">return</span> <span class="k">new</span> <span class="n">SecureRandom</span><span class="o">();</span> 
        <span class="o">}</span>
    <span class="o">}</span>
       

    <span class="cm">/**
</span><span class="cm">              * 生成指定位数的随机字串()
</span><span class="cm">     * @param size
</span><span class="cm">     * @return
</span><span class="cm">     */</span>
    <span class="kd">private</span> <span class="kd">static</span> <span class="n">String</span> <span class="nf">generateRandomAlphanumericString</span><span class="o">(</span><span class="kt">int</span> <span class="n">size</span><span class="o">)</span> <span class="o">{</span>
        <span class="k">return</span> <span class="n">RandomStringUtils</span><span class="o">.</span><span class="na">random</span><span class="o">(</span><span class="n">size</span><span class="o">,</span> <span class="n">0</span><span class="o">,</span> <span class="n">0</span><span class="o">,</span> <span class="kc">true</span><span class="o">,</span> <span class="kc">true</span><span class="o">,</span> <span class="kc">null</span><span class="o">,</span> <span class="n">SECURE_RANDOM</span><span class="o">);</span>
    <span class="o">}</span>
    
	
	<span class="cm">/**
</span><span class="cm">	   *  生成6位验证码
</span><span class="cm">	 * @return
</span><span class="cm">	 */</span>
	<span class="kd">private</span> <span class="kd">static</span> <span class="n">String</span> <span class="nf">getRandom6</span><span class="o">(){</span>
		<span class="kt">int</span> <span class="n">verifiCode</span> <span class="o">=</span> <span class="o">(</span><span class="kt">int</span><span class="o">)</span><span class="n">Math</span><span class="o">.</span><span class="na">ceil</span><span class="o">(</span><span class="n">SECURE_RANDOM</span><span class="o">.</span><span class="na">nextFloat</span><span class="o">()*</span><span class="n">1000000</span><span class="o">);</span>
		<span class="n">String</span> <span class="n">verifiCodeStr</span> <span class="o">=</span> <span class="n">String</span><span class="o">.</span><span class="na">valueOf</span><span class="o">(</span><span class="n">verifiCode</span><span class="o">);</span>
                <span class="c1">//处理产生的随机数不及6位的情况
</span><span class="c1"></span>		<span class="k">while</span><span class="o">(</span><span class="n">verifiCodeStr</span><span class="o">.</span><span class="na">length</span><span class="o">()&lt;</span><span class="n">6</span><span class="o">){</span>
			<span class="n">verifiCode</span> <span class="o">=</span> <span class="o">(</span><span class="kt">int</span><span class="o">)</span><span class="n">Math</span><span class="o">.</span><span class="na">ceil</span><span class="o">(</span><span class="n">SECURE_RANDOM</span><span class="o">.</span><span class="na">nextFloat</span><span class="o">()*</span><span class="n">1000000</span><span class="o">);</span>
			<span class="n">verifiCodeStr</span> <span class="o">=</span> <span class="n">String</span><span class="o">.</span><span class="na">valueOf</span><span class="o">(</span><span class="n">verifiCode</span><span class="o">);</span>
		<span class="o">}</span>
		<span class="k">return</span> <span class="n">verifiCodeStr</span><span class="o">;</span>
	<span class="o">}</span>
	
<span class="o">}</span>

</code></pre></td></tr></table>
</div>
</div><h2 id="问题">问题</h2>
<p>网上看到有人踩过相关的坑，SecureRandom使用不当会引起的线程阻塞
<a href="https://www.cnblogs.com/it-deepinmind/p/13344553.html">https://www.cnblogs.com/it-deepinmind/p/13344553.html</a></p>
<h3 id="原因">原因</h3>
<blockquote>
<p>SecureRandom.getInstanceStrong(); 是jdk1.8里新增的加强版随机数实现.<br>
如果你的服务器在Linux操作系统上，这里的罪魁祸首是SecureRandom generateSeed（）。它使用/dev/random生成种子。但是/dev/random是一个阻塞数字生成器，如果它没有足够的随机数据提供，它就一直等，这迫使JVM等待。键盘和鼠标输入以及磁盘活动可以产生所需的随机性或熵。但在一个服务器缺乏这样的活动，可能会出现问题</p>
</blockquote>
<h3 id="解决方案">解决方案</h3>
<blockquote>
<ol>
<li>在Tomcat环境中解决：</li>
</ol>
<p>可以通过配置 JRE 使用非阻塞的 Entropy Source：
在 catalina.sh 中加入这么一行：-Djava.security.egd=file:/dev/./urandom 即可。</p>
<ol start="2">
<li>在 JVM 环境中解决：</li>
</ol>
<p>打开jdk安装路径 $JAVA_PATH/jre/lib/security/java.security 这个文件，找到下面的内容：
securerandom.source=file:/dev/random
替换成：
securerandom.source=file:/dev/./urandom</p>
</blockquote>

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